The D1 receptor (D1R) regulates ion transport in the renal proximal tubule, thick ascending limb, and other nephron segments. In the previous cycle, we showed that in the basal state, D1R exits as oligomers in lipid rafts;agonist stimulation increases the amount of D1R monomers. Dcreasing caveolin expression or cell cholesterol impairs D1R function. (GPCR) kinase 4 (GRK4) to a greater extent than GRK2, desensitizes renal D1R. In human renal proximal tubules, only 50% of D1R is internalized following agonist occupation;desensitization may also occur at the plasma membrane. Phosphorylation, desensitization, and intemali- zation cannot be directly equated. There are many studies on the internalization of D1 R, but there are few studies on the recycling of 01R to the plasma membrane. It is known that a sorting signal in the carboxy end of D1R is important in the endocytosis and recycling of D1R but the protein(s) responsible for D1R internali- zation and recycling are not known. We have evidence that sorting nexinS (SNX5) and SNX1 (determined by biochemical methods, confocal microscopy, Foster resonance energy transfer, and fluorescence life time imaging) are involved in D1R and D5R trafficking, respectively. Protein phosphatase 2A (PP2A) is also res- ponsible, in part, for the resensitization of D1R but the mechanism and the subcellular site of PP2A-mediated resensitization of D1R are unknown. Studying the mechanisms and pathways by which D1R and 05R are trafficked in human renal proximal tubule cells, in vitro, and in rats in vivo is the main objective which will be tested by two related hypotheses. Specific aim 1 is a series of cell and tubule studies designed to test the hypothesis that SNX5 regulates the endocytosis and cell surface membrane recycling of D1R while SNX1 regulates D5R. Specific aim 2 is also be a series of cell and tubule studies designed to test the hypothesis that PP2A regulates the cell surface membrane recycling of D1R and D5R. Specific aim 3 will test the hypothesis that renal SNX1, SNX5, and PP2A regulate D1R and D5R function in vivo by chronically inhibiting the renal cortical expression of SNX5, SNX1, or PP2A in the presence or absence of expression either D1R or D5R in conscious rats. Impaired renal D1R function is found in humans with essential hypertension and in animal models of genetic hypertension. Disruption of the 01R gene causes hypertension in mice. Therefore, the proposed studies may lead to a better understanding of the